1. Field of the Invention
The present invention relates to a transmission including a multiple planetary mechanism into which power from a driving source is inputted through an input shaft.
2. Description of the Related Art
Published Japanese Translation No. 2008-531954 of PCT/EP2006/001937 has made known a transmission which achieves eight selectable forward gear ratios and a single reverse gear ratio by use of four planetary mechanisms and five engagement components. In addition, Japanese Patent Application Laid-open No. 11-51125 has made known a twin clutch transmission which achieves multiple speeds by use of a twin clutch.
In the transmission disclosed by Published Japanese Translation No. 2008-531954 of PCT/EP2006/001937. each engagement component is used for multiple transmission speeds. Because a torque intended to be transmitted varies depending on the transmission speeds, each engagement component needs to be designed corresponding to a transmission speed for transmitting the largest torque. This makes the transmission control difficult. To put it specifically, in some cases, it is necessary to control a small torque with a clutch having a large capacity, as if watering a flower garden with a fire pump. In addition, while idling, some engagement component may cause a large difference in rotation of the planetary mechanisms. This unnecessarily increases the friction between the planetary mechanisms. In the worst case, this causes troubles such as sticking in the transmission, thus resulting in deterioration in efficiency of the transmission.
Such a transmission as disclosed by Japanese Patent Application Laid-open No. 11-51125 is generally of a twin-counter type which uses two or more countershafts for the purpose of making shorter the overall length of the main shaft, namely the overall length of the transmission with consideration being given to the mounting of a transverse engine on a front-drive vehicle. In this case, the countershafts, which are installed under the main shaft and the differential shaft, rotate in oil. This increases the resistance to stirring and thus causes the deterioration of the transmission efficiency.
Further, in a case where a transmission having 8 or more transmission speeds is needed in view of a demand for improvement in fuel mileage, and the like, the number of gear trains should be increased. This makes longer the overall length of the main shaft, namely the overall length of the transmission. This elongation accordingly impairs the mountability of the transmission onto the vehicle. For this reason, it is difficult to achieve the transmission having a larger number of transmission speeds.
Further, in a case where a span of any shaft which is supported by a bearing is long, the shaft is apt to deform when heavily loaded, and the durability and the like is apt to be adversely affected. If the rigidity of the shaft is increased as countermeasures, the weight of the transmission conversely increases, and the fuel mileage accordingly decreases. For this reason, it is difficult to mount such a transmission on a vehicle which is expected to be driven with a heavy load such as the hauling of a trailer.
Furthermore, with regard to transmission control, the transmission disclosed by Japanese Patent Application Laid-open No. 11-51125 is not capable of smoothly changing transmission speeds one-by-one (from an odd-numbered speed to an even-numbered speed, or from an even-numbered speed to an odd-numbered speed) because of its structural characteristics. Moreover, when a driver unexpectedly shifts the transmission to a higher transmission gear or a lower transmission gear, the shift needs to be achieved by a preliminary shift in a high-speed mode. In addition, in order to secure a satisfactory response time for the gear shift, a synchronization mechanism with a larger capacity is needed. This increases friction.